Our understanding of how developing neurons express the neurotransmitter(s) appropriate for synaptic communication is surprisingly limited. Target-derived signals are known to alter transmitter expression in developing autonomic neurons. To address transmitter regulation in the developing CNS, where analysis of neuron-target interaction is more difficult, the proposed studies will explore the phenomenon of transient transmitter expression. Previous work demonstrated that certain transmitters are present in subpopulations of neurons in specific brain regions of developing rats, but these transmitters are not seen in these same areas of adult brains. A subset of neurons in the developing rat cerebral cortex, for example, transiently displays immunoreactivity for tyrosine hydroxylase (TH), a catecholamine-synthetic enzyme. One explanation for the transient observation of phenotypic markers is that these neurons undergo programmed cell death (apoptosis). Alternatively, these cells may transition from the production of one transmitter to another during development. To test these hypotheses, the proposed studies will: I. Determine if the cortical neurons that appear to transiently express TH undergo apoptosis during development. Sections of rat brains at ages during which transient TH immunoreactivity was reported will be double-labeled for TH and markers of apoptosis and examined by fluorescence microscopy. Fragmented DNA in apoptotic cells will be visualized by attaching fluorescent nucleotides to DNA termini (TUNEL). The pro-apoptotic active caspase-3 protease will be detected immunohistochemically. II. Determine if developing CNS neurons switch their transmitter production. Brain sections will be immunolabeled for both TH and known cortical transmitters to co-localize dual phenotypes that may be present during the transition. The presence of apoptotic markers in TH neurons would suggest the selective death of these cells, explaining their apparent phenotypic transience. If the TH cells are not apoptotic and TH is co-localized with markers for other transmitters, then it is possible that some developing CNS neurons exhibit plasticity with respect to their transmitter production.